Quantitative Measurement And Physical Nature Of The Spring Entropic Effect In LCST Phase Separation Of Polymer Aqueous Solutions

The lower critical solution temperature(LCST)phase separation behaviors of thermosensitive poly(vinyl methyl ether)(PVME)and poly(N-isopropylacrylamide)(PNIPAM)aqueous solutions were studied by power-compensation differential scanning calorimetry(DSC),respectively.The interaction between spring entropy and hydrogen bond in phase separation of PVME aqueous solution was quantitatively analyzedby investigating the isothermal recovery behaviors of demixing enthalpy in different concentration regions.The thermal effect contributed by hydrogen bond interaction between the oxygen ether group and water molecule in PVME aqueous solutions is almost constant in the process of phase separation,while the thermal effect contributed by spring entropy varies with the water content.The thermal effect induced by spring entropy is zero in the region when water concentration is lower than 38.3 wt%,which is corresponding to a compact globule state of PVME coil.At a high water concentration region(W_water>38.3 wt%),the spring entropic effect significantly increases with the increasing of water content.Futhermore,the results also show that the contribution of the entropic effect exceeds the enthalpic effect contributed by hydrogen bonding when water content is above 65 wt%,which confirms de Gennes'prediction that the entropic effect in the soft matter system is even greater than the enthalpic effect.Based on the measured results mentioned above,in this work we innovatively use energetic change as the standard to probe the relative conformational coil-to-globule transition of polymer chains in concentrated LCST aqueous polymer solutions,quantatively determined the entropic effects and hence deduced the change of conformational swelling states of polymer solutions.Based on the preannealing results effect on the entropic effect at different temperatures,it is found that the dual entropic and enthalpic processes exist in the phase separation of PVME aqueous solution.Specifically,the entropic process corresponding to the collapse of the polymer coils appears in a very narrow theta temperature range,which corresponds to the changes of water from the good to theta,and eventually to a poor solvent for PVME.On the contrary,the enthalpic process takes place in a wide temperature range,corresponding to the process that the hydrogen bond interaction between water and PVME decreases gradually with the increase of temperature.The recovery of enthalpy of isothermal phase separation generated by the entropy effect was also observed in another LCST PNIPAM aqueous system,the entropy effect and its indicating on the change of swelling state in aqueous solution were further clarified.Furthermore,the effect of NH₄Cl on the phase separation of PVME aqueous solution was also studied.It turned out that the existence of NH₄Cl not only significantly affects the LCST phase separation temperature of polymer solution,but also results in a substantial change of the theta temperature region corresponding to the collapse of polymer coils.In the mean time,the entropic effect in PVME aqueous solution with NH₄Cl is more significant than that in pure water systems.In addition,in the investigation of temperature dependence of absolute heat capacity in the polyolefins with different architectures,it is found that the influence of different architectures on the absolute heat capacity is very significant,which indicates that the measurement of thermodynamic properties such as absolute heat capacity is useful to have more comprehensive understand on the entropic and enthalpic effects in the crystallization of polymers with complicate architectures.